6,610 research outputs found
A Comparison of Synthetic Seismograms for 2D Structures: Semianalytical versus Numerical
Teleseismic wave fields occasionally exhibit rapid changes in travel times and waveforms over distances less than several great-circle degrees when observed at broadband arrays. These rapid changes in wave field suggest the existence of significant structural transitions occurring over scales of several hundred kilometers or less in the mid- and deep mantle. Although approximate analytical methods based on raytracing can be readily adapted to structures having arbitrarily small scale lengths, it is important to validate their accuracy against the predictions of numerical methods. Here we compare synthetics from an approximate ray-based method WKBJ modified (WKM) against the pseudospectral method for a 2D model of the S-velocity anomaly associated with the South African plume. This model consists of a uniform 3% decrease in S velocity over a broad (>10°) region of the mid- and deep mantle beneath South Africa, contiguous at its bottom with a thin (100- to 200-km-thick) zone of low velocity extending 30° westward toward South America along the core-mantle boundary. Transitions between anomalous and radially symmetric structures of the test model are sharp, occurring over l0 km or less. SV and SH wave fields synthesized by the WKM and pseudospectral methods in this model generally agree with each other well. Slight mismatches in the two methods can be understood as the result of either differences in model parameterization or the effects of asymptotic approximations in the ray-based WKM method
Structure and dynamics of Oxide Melts and Glasses : a view from multinuclear and high temperature NMR
Solid State Nuclear Magnetic Resonance (NMR) experiments allow characterizing
the local structure and dynamics of oxide glasses and melts. Thanks to the
development of new experiments, it now becomes possible to evidence not only
the details of the coordination state of the network formers of glasses but
also to characterize the nature of polyatomic molecular motifs extending over
several chemical bonds. We present results involving 31P homonuclear
experiments that allow description of groups of up to three phosphate units and
27Al/17O heteronuclear that allows evidencing μ3 oxygen bridges in
aluminate glasses and rediscussion of the structure of high temperature melts.Comment: Journal of Non-Crystalline Solids (2007) in press; Also available
online at: http://crmht.cnrs-orleans.fr/Intranet/Publications/?id=207
A milestone toward understanding PDR properties in the extreme environment of LMC-30Dor
More complete knowledge of galaxy evolution requires understanding the
process of star formation and interaction between the interstellar radiation
field and the interstellar medium in galactic environments traversing a wide
range of physical parameter space. Here we focus on the impact of massive star
formation on the surrounding low metallicity ISM in 30 Doradus in the Large
Magellanic Cloud. A low metal abundance, as is the case of some galaxies of the
early universe, results in less ultra-violet shielding for the formation of the
molecular gas necessary for star formation to proceed. The half-solar
metallicity gas in this region is strongly irradiated by the super star cluster
R136, making it an ideal laboratory to study the structure of the ISM in an
extreme environment. Our spatially resolved study investigates the gas heating
and cooling mechanisms, particularly in the photo-dissociation regions where
the chemistry and thermal balance are regulated by far-ultraviolet photons (6
eV< h\nu <13.6 eV).
We present Herschel observations of far-infrared fine-structure lines
obtained with PACS and SPIRE/FTS. We have combined atomic fine-structure lines
from Herschel and Spitzer observations with ground-based CO data to provide
diagnostics on the properties and the structure of the gas by modeling it with
the Meudon PDR code. We derive the spatial distribution of the radiation field,
the pressure, the size, and the filling factor of the photodissociated gas and
molecular clouds. We find a range of pressure of ~ 10^5 - 1.7x10^6 cm^{-3} K
and a range of incident radiation field G_UV ~ 10^2 - 2.5x10^4 through PDR
modeling. Assuming a plane-parallel geometry and a uniform medium, we find a
total extinction of 1-3 mag , which correspond to a PDR cloud size of 0.2 to
3pc, with small CO depth scale of 0.06 to 0.5pc. We also determine the three
dimensional structure of the gas. (Abridged)Comment: 20 pages, 23 figures, accepted in A&
Physical conditions in the gas phases of the giant HII region LMC-N11 unveiled by Herschel - I. Diffuse [CII] and [OIII] emission in LMC-N11B
(Abridged) The Magellanic Clouds provide a nearby laboratory for metal-poor
dwarf galaxies. The low dust abundance enhances the penetration of UV photons
into the interstellar medium (ISM), resulting in a relatively larger filling
factor of the ionized gas. Furthermore, there is likely a hidden molecular gas
reservoir probed by the [CII]157um line. We present Herschel/PACS maps in
several tracers, [CII], [OI]63um,145um, [NII]122um, [NIII]57um, and [OIII]88um
in the HII region N11B in the Large Magellanic Cloud. Halpha and [OIII]5007A
images were used as complementary data to investigate the effect of dust
extinction. Observations were interpreted with photoionization models to infer
the gas conditions and estimate the ionized gas contribution to the [CII]
emission. Photodissociation regions (PDRs) are probed through polycyclic
aromatic hydrocarbons (PAHs). We first study the distribution and properties of
the ionized gas. We then constrain the origin of [CII]157um by comparing to
tracers of the low-excitation ionized gas and of PDRs. [OIII] is dominated by
extended emission from the high-excitation diffuse ionized gas; it is the
brightest far-infrared line, ~4 times brighter than [CII]. The extent of the
[OIII] emission suggests that the medium is rather fragmented, allowing far-UV
photons to permeate into the ISM to scales of >30pc. Furthermore, by comparing
[CII] with [NII], we find that 95% of [CII] arises in PDRs, except toward the
stellar cluster for which as much as 15% could arise in the ionized gas. We
find a remarkable correlation between [CII]+[OI] and PAH emission, with [CII]
dominating the cooling in diffuse PDRs and [OI] dominating in the densest PDRs.
The combination of [CII] and [OI] provides a proxy for the total gas cooling in
PDRs. Our results suggest that PAH emission describes better the PDR gas
heating as compared to the total infrared emission.Comment: Accepted for publication in Astronomy and Astrophysics. Fixed
inverted line ratio in Sect. 5.
Recommended from our members
Evidence for back scattering of near-podal seismic P'P' waves from the 150-220 km zone in Earth's upper mantle
The deepest and most inaccessible parts of Earth's interior--the core and core-mantle boundary regions can be studied from compressional waves that turn in the core and are routinely observed following large earthquakes at epicentral distances between 145{sup o} and 180{sup o} (also called P', PKIKP or PKP waves). P'P' (PKPPKP) are P' waves that travel from a hypocenter through the Earth's core, reflect from the free surface and travel back through the core to a recording station on the surface. P'P' waves are sometimes accompanied by precursors, which were reported first in the 1960s as small-amplitude arrivals on seismograms at epicentral distances of about 50{sup o}-70{sup o}. Most prominent of these observed precursors were explained by P'P' waves generated by earthquakes or explosions that did not reach the Earth's surface but were reflected from the underside of first order velocity discontinuities at 410 and 660 km in the upper mantle mantle. Here we report the discovery of hitherto unobserved near-podal P'P' waves (at epicentral distance less than 10{sup o}) and very prominent precursors preceding the main energy by as much as 55 seconds. We interpret these precursors as a back scattered energy from undocumented structure in the upper mantle, in a zone between 150 and 220 km depth beneath Earth's surface. From these observations, we identify a frequency dependence of Q (attenuation quality factor) in the lithosphere that can be modeled by a flat relaxation spectrum below about 0.05-0.1 Hz and increasing with as the first power of frequency above this value, confirming pioneering work by B. Gutenberg
The effects of star formation on the low-metallicity ISM: NGC4214 mapped with Herschel/PACS spectroscopy
We present Herschel/PACS spectroscopic maps of the dwarf galaxy NC4214
observed in 6 far infrared fine-structure lines: [C II] 158mu, [O III] 88mu, [O
I] 63mu, [O I] 146mu, [N II] 122mu, and [N II] 205mu. The maps are sampled to
the full telescope spatial resolution and reveal unprecedented detail on ~ 150
pc size scales. We detect [C II] emission over the whole mapped area, [O III]
being the most luminous FIR line. The ratio of [O III]/[C II] peaks at about 2
toward the sites of massive star formation, higher than ratios seen in dusty
starburst galaxies. The [C II]/CO ratios are 20 000 to 70 000 toward the 2
massive clusters, which are at least an order of magnitude larger than spiral
or dusty starbursts, and cannot be reconciled with single-slab PDR models.
Toward the 2 massive star-forming regions, we find that L[CII] is 0.5 to 0.8%
of the LTIR . All of the lines together contribute up to 2% of LTIR . These
extreme findings are a consequence of the lower metallicity and young,
massive-star formation commonly found in dwarf galaxies. These conditions
promote large-scale photodissociation into the molecular reservoir, which is
evident in the FIR line ratios. This illustrates the necessity to move to
multiphase models applicable to star-forming clusters or galaxies as a whole.Comment: Accepted for publication in the A&A Herschel Special Issu
Linking dust emission to fundamental properties in galaxies: The low-metallicity picture
In this work, we aim at providing a consistent analysis of the dust
properties from metal-poor to metal-rich environments by linking them to
fundamental galactic parameters. We consider two samples of galaxies: the Dwarf
Galaxy Survey (DGS) and KINGFISH, totalling 109 galaxies, spanning almost 2 dex
in metallicity. We collect infrared (IR) to submillimetre (submm) data for both
samples and present the complete data set for the DGS sample. We model the
observed spectral energy distributions (SED) with a physically-motivated dust
model to access the dust properties. Using a different SED model (modified
blackbody), dust composition (amorphous carbon), or wavelength coverage at
submm wavelengths results in differences in the dust mass estimate of a factor
two to three, showing that this parameter is subject to non-negligible
systematic modelling uncertainties. For eight galaxies in our sample, we find a
rather small excess at 500 microns (< 1.5 sigma). We find that the dust SED of
low-metallicity galaxies is broader and peaks at shorter wavelengths compared
to more metal-rich systems, a sign of a clumpier medium in dwarf galaxies. The
PAH mass fraction and the dust temperature distribution are found to be driven
mostly by the specific star-formation rate, SSFR, with secondary effects from
metallicity. The correlations between metallicity and dust mass or total-IR
luminosity are direct consequences of the stellar mass-metallicity relation.
The dust-to-stellar mass ratios of metal-rich sources follow the well-studied
trend of decreasing ratio for decreasing SSFR. The relation is more complex for
highly star-forming low-metallicity galaxies and depends on the chemical
evolutionary stage of the source (i.e., gas-to-dust mass ratio). Dust growth
processes in the ISM play a key role in the dust mass build-up with respect to
the stellar content at high SSFR and low metallicity. (abridged)Comment: 44 pages (20 pages main body plus 5 Appendices), 11 figures, 9
tables, accepted for publication in A&
On the gravitational production of superheavy dark matter
The dark matter in the universe can be in the form of a superheavy matter
species (WIMPZILLA). Several mechanisms have been proposed for the production
of WIMPZILLA particles during or immediately following the inflationary epoch.
Perhaps the most attractive mechanism is through gravitational particle
production, where particles are produced simply as a result of the expansion of
the universe. In this paper we present a detailed numerical calculation of
WIMPZILLA gravitational production in hybrid-inflation models and
natural-inflation models. Generalizing these findings, we also explore the
dependence of the gravitational production mechanism on various models of
inflation. We show that superheavy dark matter production seems to be robust,
with Omega_X h^2 ~ (M_X / (10^11 GeV))^2 (T_RH / (10^9 GeV)), so long as M_X <
H_I, where M_X is the WIMPZILLA mass, T_RH is the reheat temperature, and H_I
is the expansion rate of the universe during inflation.Comment: 26 pages, 7 figures; LaTeX; submitted to Physical Review D; minor
typographical error correcte
Time-dependent calculation of ionization in Potassium at mid-infrared wavelengths
We study the dynamics of the Potassium atom in the mid-infrared, high
intensity, short laser pulse regime. We ascertain numerical convergence by
comparing the results obtained by the direct expansion of the time-dependent
Schroedinger equation onto B-Splines, to those obtained by the eigenbasis
expansion method. We present ionization curves in the 12-, 13-, and 14-photon
ionization range for Potassium. The ionization curve of a scaled system, namely
Hydrogen starting from the 2s, is compared to the 12-photon results. In the
13-photon regime, a dynamic resonance is found and analyzed in some detail. The
results for all wavelengths and intensities, including Hydrogen, display a
clear plateau in the peak-heights of the low energy part of the Above Threshold
Ionization (ATI) spectrum, which scales with the ponderomotive energy Up, and
extends to 2.8 +- 0.5 Up.Comment: 15 two-column pages with 15 figures, 3 tables. Accepted for
publication in Phys. Rev A. Improved figures, language and punctuation, and
made minor corrections. We also added a comparison to the ADK theor
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